Vidaara.orgClass 9 · Chemistry
CodeVID-C9-04-T2-01
Assignment — Bohr's Model & Electronic Configuration
Name: ____________________
Roll No.: __________
Date: ____________
General Instructions
- All questions are compulsory.
- Section A carries 1 mark each, Section B 2 marks, Section C 3 marks and Section D 5 marks.
- Show all working for Sections B, C and D. Only final answers are given at the end — for full solutions, raise your doubts with your teacher.
Section A — Multiple Choice Questions
5 × 1 = 5 marks
1.
The shell closest to the nucleus is called:
- A.L shell
- B.M shell
- C.K shell
- D.N shell
2.
The maximum number of electrons in the L shell is:
- A.2
- B.8
- C.18
- D.32
3.
The formula for the maximum electrons in a shell is:
- A.2n
- B.n²
- C.2n²
- D.n/2
4.
The electronic configuration of magnesium (Z = 12) is:
- A.2, 8, 2
- B.2, 10
- C.2, 8, 4
- D.8, 2, 2
5.
Bohr's model successfully explained the atom's:
- A.colour
- B.stability
- C.smell
- D.weight only
Section B — Short Answer (2 marks)
3 × 2 = 6 marks
6.
State any two postulates of Bohr's model of the atom.
7.
Write the electronic configurations of oxygen (Z = 8) and chlorine (Z = 17).
8.
What is the maximum number of electrons in the outermost shell, and why is this rule needed?
Section C — Short Answer (3 marks)
2 × 3 = 6 marks
9.
Using the 2n² rule, find the maximum electrons in the K, L, M and N shells.
10.
Write the electronic configuration of calcium (Z = 20) and explain why the M shell does not take 10 electrons.
Section D — Long Answer (5 marks)
1 × 5 = 5 marks
11.
State Bohr's postulates and use them, with the 2n² and outermost-shell rules, to write the electronic configurations of potassium (Z = 19) and argon (Z = 18).
Answer Key
Section A — Multiple Choice Questions
- (C) K shell
- (B) 8
- (C) 2n²
- (A) 2, 8, 2
- (B) stability
Section B — Short Answer (2 marks)
- Electrons revolve only in certain fixed circular orbits (shells/energy levels), and they do not radiate energy while staying in a shell; energy changes only when an electron jumps between shells.
- Oxygen (Z = 8): 2, 6. Chlorine (Z = 17): 2, 8, 7.
- The outermost shell can hold at most 8 electrons; this rule, together with stepwise filling, explains the configurations of the first 18–20 elements.
Section C — Short Answer (3 marks)
- K (n=1) = 2×1² = 2; L (n=2) = 2×2² = 8; M (n=3) = 2×3² = 18; N (n=4) = 2×4² = 32.
- Ca (Z = 20): 2, 8, 8, 2. After L is full, the M shell would be the outermost, so it can take only 8 (octet rule) before the next two electrons begin the N shell.
Section D — Long Answer (5 marks)
- Postulates: electrons revolve in fixed circular orbits called shells/energy levels of definite energy; they do not radiate energy while in a shell, so the atom is stable; energy is absorbed or released only on jumping between shells. Filling rules: a shell holds at most 2n² electrons, the outermost shell holds at most 8, and inner shells fill first. Argon (Z = 18): 2, 8, 8. Potassium (Z = 19): after 2, 8, the M shell would be outermost so it takes only 8, and the 19th electron begins the N shell, giving 2, 8, 8, 1.
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